A common form of pointing device for use with computers and the like is referred to as a “mouse”. The computer user moves the device over a surface to move a cursor on the computer screen. The amount and direction of motion of the mouse are sensed by the mouse and determine the distance and direction in which the cursor moves on the screen. Inexpensive mice based on a ball that rolls over the surface have been used for some time. The ball turns two cylinders that sense the distance and direction of motion. Unfortunately, the ball picks up grease and other dirt from the surface and transfers this material to the cylinders. The resulting coating on the cylinders interferes with the motion of the cylinders, and hence, the devices must be cleaned periodically. The cleaning operation is awkward and time consuming. In addition, the numerous mechanical assemblies included in the device increase the cost of assembly.
Mice based on optical sensing avoid this problem. Originally, such mice had to be moved over a special pad that had grid lines that were sensed by the device. The need to use this special pad made these devices less attractive than the mechanical mice discussed above. More recently, optical mice that do not require such pads have been developed. These mice include a light source that illuminates the surface under the mouse at a shallow angle, which accentuates the structural details of the surface. An image sensor in the mouse records an image of the illuminated surface periodically. By comparing two successive images, the displacement of the mouse between the times at which the images were taken can be determined.
Cost and power consumption are particularly important factors in the design of an optical mouse. Optical mice are often powered by batteries, either directly or indirectly through a serial link to a laptop computer that is run on batteries. Hence, the power available to the mouse is often limited. Furthermore, the need to replace batteries in a mouse is viewed negatively by many consumers.
Optical mice typically utilize a red light source such as a red LED and a silicon image sensor. The silicon image sensor is inexpensive. Silicon image sensors have significantly greater light conversion efficiency in the red region of the spectrum. In addition, high efficiency red LEDs are relatively inexpensive. Hence, this combination provides an inexpensive mouse that requires relatively low power levels to operate provided the surface over which the mouse is moved reflects red light with high efficiency.
The signal generated by each pixel in the image sensor is proportional to the light received by that pixel times the conversion efficiency of the silicon pixel at the wavelength of the light generated by the light source. The efficiency of silicon for a red light is typically about a factor of two higher than the efficiency of silicon for a blue light.
Consider the case of a red surface that is illuminated by a red light source. Denote the signal generated by the silicon image sensor in this case by 1 unit. If a blue light source is used to illuminate a blue surface, the signal value would be only about 0.5 units, since the silicon pixels ability to convert the light to an electrical signal is only half that of the conversion efficiency for a red light. However, if a red light source is used to illuminate a blue surface, the signals from the pixels will be a small fraction of a unit, since most of the red light will be absorbed by the blue surface, and hence, not reach the image sensor. Hence, the performance of the mouse is also dependent on the color of the surface over which the mouse navigates, and this factor can be more important than the difference in conversion efficiency of the silicon sensor as a function of wavelength.
In principle, this problem can be reduced by increasing the intensity of the illumination source to increase the amount of light reaching the sensor when the navigation surface has an unfavorable color. However, this would increase the power expended by the mouse. In battery powered devices, the increased power requirements can present a significant problem. In addition, the life of the LED would be reduced by the increased power.